Crazy easy. I just bought one and rooted the next day.
Download the drivers
USB and PC Charging Drivers - Motorola Mobility, Inc. USA
Download gingerbreak
GingerBreak.exe - 4shared.com - online file sharing and storage - download
Make sure you have a microSD card installed/enable usb debugging mode-Menu~Settings~Applications~Development~Enable USB Debugging
Place USB connection mode to "None"
Run Gingerbreak (if the drivers were installed properly, gingerbreak will show up on your Atrix)
Run the Gingerbreak app/click root device (it'll reboot on it's own)
You're rooted! Go to your apps and you should see "Superuser".